Nelson River Pole 1 Thyristor Leakage Problem

The Nelson River HVDC system currently consists of two thyristor-based bipole schemes: Bipoles I and II. Pole 1 in Bipole I was found to have suffered from a serious thyristor leakage problem (TLP) after 20 years in service. In order to understand, evaluate, and resolve this problem, a comprehensive study was conducted in 2013 at Manitoba Hydro, which involved thyristor voltage monitoring, leakage current measurements, and simulations. The study revealed that the majority of thyristors in two of the three valve groups at both ends were leaky (i.e., exhibiting increased leakage currents) to various degrees. This paper provides an overview of the TLP and related investigation. The effects and symptoms/behaviors of the leaky thyristors are illustrated with the study results for one of the worst valves at each end. The failure mechanism of the leaky thyristors is also discussed. This paper should provide a clear understanding of the TLP from the user's perspective. A question, however, remains with regard to the root-cause of the TLP. Further research is required to answer this question.